Modern depictions are not far from the truth. Of those professional Jesters (Licensed Fools) that we know of, many of them did enjoy the privilege of free speech and the ability to criticize their patrons and their guests much like the Shakespearean Fools in King Lear, Twelfth Night, etc.

Although largely insulated from being punished for speech, there were lines that when crossed could be perilous for the performer. For example, William Sommers (Jester to King Henry VIII of England) was threatened with death at the king's own hand after he referred to the Princess Elizabeth (later Queen Elizabeth I) as a bastard.

As for famous documented Jesters other than Sommers:

• Stańczyk: Legendary & mysterious Polish Jester and symbol of Polish identity.

• Jeffrey Hudson: Court Dwarf to Queen Henrietta Maria of France.

• Taillefer: Part of the train of William the Conquerer. Torn to pieces at the Battle of Hastings.

If you want to know a little more, I talk about Jesters here, here, and here. If you want to know a lot more, read Fools Are Everywhere: The Court Jester Around the World by Beatrice K. Otto.